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Probabilistic
Seismic Hazard Mapping with Site
Amplification Effects
An automated
seismic hazard
software was developed to define probabilistic seismic hazard maps by
performing multiple analyses
over a grid of pre-defined locations. The analyses account for site
amplification effects based on the often imperfect knowledge of local
soil conditions at each location.
The system is
composed of three
different modules.
The site
characterisation module
is used to infer the most likely
soil properties at each location based on the available information.
The code treats the input data in a consistent manner via pre-defined
and transparent rules. If the data are scarce (e.g.,
geologic maps only), the rules define different possible alternative
configurations and assign a weight to each of them. This module
provides the necessary surface soil information for the site
amplification module.
The seismic
hazard module computes
the seismic hazard on rock at
each node. The module considers multiple hypotheses on the input
assumptions to the seismicity model (e.g., the locations and other
characteristics of seismic sources, such as the activity rate) via a
logic tree approach. The module is able to handle both plane sources
(i.e., faults) and volumetric sources (e.g., seismotectonic
provinces). As a by-product this module also perform seismic
disaggregation in magnitude and source-to-site distance to identify
potential earthquake scenarios that control the hazard at each site.
This module produces the seismic hazard maps assuming bedrock
conditions everywhere by interpolating the results at each node.
The site
amplification module
computes amplification functions
for each site based on the soil characterization provided by the
site characterisation module. The amplification of different ground
motion parameters (e.g., peak ground acceleration and spectral
quantities) is established at each site by means of a response
surface technique that uses pre-computed amplification functions
derived for a large database of soil columns. The amplification
functions were derived using an advanced nonlinear finite element
code that considers the effects of earthquake-induced cyclic mobility
and liquefaction. Multiple alternatives are analysed via a logic tree
approach and the epistemic uncertainty in the input soil formation is
appropriately propagated to the final estimates of the surface
hazard.
At each
location the hazard at the
ground surface is determined
by probabilistically combining the seismic hazard at the bedrock
(obtained by the seismic hazard module) and the amplification
functions (obtained by site amplification analyses) adopting a
convolution technique.
As an example
of the use of this
tool, both a
bedrock and a surface hazard maps for an area in Italy are presented.
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